With my daughter, if I have to say "no" or "not now" I always try to provide an explanation, which ranges from "we don't have time now because we have <some time/date/location> to get to" to "I think that's too dangerous <explanation of risks>". I've done that every since she could speak. She's 10 years old now and I still do it. That being said, I don't often tell her "no". If she wants to do something that involves risk of injury (or something similar, like making a huge mess), I walk her through what she wants to do and get her to see where she needs to be proactive. She doesn't always listen.
For example (when she was younger), I have a four-wheeled cart for moving things which she wanted to ride in the driveway. I pointed out that she definitely would fall off and land on the concrete at some point, which made her understand she needed to wear protective gear (knee/elbow pads, wrist guards, and a helmet... basically skating/skateboarding gear). Of course, one time I came home from work and my wife told me my daughter needed to tell me something. That something was that she rode the cart without the gear and fell off and injured her hands and her chin by falling face first into the concrete (luckily it was only a minor injury). I don't think she ever tried to ride the cart again, though I wasn't opposed to it (with the proper gear of course).
This is just an anecdote to sort of "illustrate the process" I suppose. I hope it is at least moderately useful. And this makes it sound easy, which it is definitely not. It's tough, but so far it seems to have worked well.
As opposed to "no" or "not now" you can also say "wait a moment" or "I'll help you later". No need to explain. Just make sure there's a way forward. She just needs to hold on a bit. Giving "no" even with an explanation as to why not might not yet show a way forward.
My daughter always comes with all kinds of questions when it's bedtime (and she doesn't want to sleep). Simply answer: "Let's talk about that tomorrow. Now it's time to sleep."
For example (when she was younger), I have a four-wheeled cart for moving things which she wanted to ride in the driveway. I pointed out that she definitely would fall off and land on the concrete at some point, which made her understand she needed to wear protective gear (knee/elbow pads, wrist guards, and a helmet... basically skating/skateboarding gear). Of course, one time I came home from work and my wife told me my daughter needed to tell me something. That something was that she rode the cart without the gear and fell off and injured her hands and her chin by falling face first into the concrete (luckily it was only a minor injury). I don't think she ever tried to ride the cart again, though I wasn't opposed to it (with the proper gear of course).
This is just an anecdote to sort of "illustrate the process" I suppose. I hope it is at least moderately useful. And this makes it sound easy, which it is definitely not. It's tough, but so far it seems to have worked well.